Compression testing of composites without anti-buckling guide plates, is often preferred as the free unsupported length is more representative of true compression behavior. This method relies on the accuracy of the alignment between the upper fixture and the lower fixture while introducing the compression load. In addition, the natural failure process of the specimen will induce high lateral forces that must be resisted to ensure that forces remain in true compression. Therefore, the principle behind the IITRI, Wyoming Modified Celanese and Combined Loading Compression fixtures is essentially the same.
Where they are different, is the way the compression load is being introduced – either by shear transfer from jaw faces to specimen, or a combination of shear and end loading. This is the primary difference with the CLC fixture where the shear load component is from the tightening of the screws which is combined with end loads introduced directly. The IITRI and Celanese rely on the self tightening effect from the wedge jaw faces. The Celanese fixture also allows for preloading the jaw faces by manual screws when the specimen is being installed, while the IITRI also includes end bars that could be used to introduce end loads to the specimen.
In terms of usability, the IITRI allows the largest specimens, up to 38 mm wide, but is the heaviest of the fixtures. The CLC is conceptually the simplest but care must be exercised to ensure the specimen is properly located and improper or uneven tightening of the clamping screws can affect results. The Celanese is a neat compact fixture but is limited to only 12.7 mm (0.5 in.) wide specimens.